In this paper, we introduce a novel technique (anomaly detection) for the online detection of impending instability in a combustion system based on symbolic time series analysis. The experimental results presented in this paper illustrate the application of anomaly detection to a combustor in which the flame is stabilized either by a bluff body or by a swirler. The detection unit works on the principle that in the transition region from combustion noise to thermoacoustic instability, combustion systems exhibit peculiar dynamics which results in the formation of specific patterns in the time series. Further, tools from symbolic time series analysis is used to recognize these patterns and then define an anomaly measure indicative of the proximity of system to regimes of thermoacoustic instability.

R. I. Sujith

Department of Aerospace Engineering

combustion

Fuel systems

Harmonic analysis

Signal detection

stability

Thermoacoustics

anomaly detection

COMBUSTION SYSTEMS

INSTABILITY DETECTIONS

ON-LINE DETECTION

Probabilistic finite state automaton

SYMBOLIC TIME SERIES ANALYSIS

Thermoacoustic instability

Transition regions

time series analysis

Fulltext

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IIT Madras

Online Detection of Impending Instability in a Combustion System Using Tools from Symbolic Time Series Analysis

International Journal of Spray and Combustion Dynamics

Thermoacoustic instability is a result of the positive feedback between the acoustic pressure and the unsteady heat release rate fluctuations in a combustor. We apply the framework of the synchronization theory to study the coupled behavior of these oscillations during the transition to thermoacoustic instability in a turbulent bluff-body stabilized gas-fired combustor. Furthermore, we characterize this complex behavior using recurrence plots and recurrence networks. We mainly found that the correlation of probability of recurrence (C P R), the joint probability of recurrence (J P R), the determinism (D E T), and the recurrence rate (R R) of the joint recurrence matrix aid in detecting the synchronization transitions in this thermoacoustic system. We noticed that C P R and D E T can uncover the occurrence of phase synchronization state, whereas J P R and R R can be used as indices to identify the occurrence of generalized synchronization (GS) state in the system. We applied measures derived from joint and cross recurrence networks and observed that the joint recurrence network measures, transitivity ratio, and joint transitivity are useful to detect GS. Furthermore, we use the directional property of the network measure, namely, cross transitivity to analyze the type of coupling existing between the acoustic field (p ′) and the heat release rate (q ′) fluctuations. We discover a possible asymmetric bidirectional coupling between q ′ and p ′, wherein q ′ is observed to exert a stronger influence on p ′ than vice versa. © 2018 Author(s).

Chaos

Coupled interaction between unsteady flame dynamics and acoustic field in a turbulent combustor

Flame-acoustic interactions are witnessed in the context of combustion instability in gas turbine combustors and other propulsion devices, such as rockets, besides confined combustion systems in general, such as furnaces and heaters. The confinement causes acoustic standing wave modes that interact with the flame to cause fluctuations in all quantities to grow in amplitude. This review categorizes the different canonical flame-holding geometries that mostly involve flow recirculation zones for flame stabilization, which are inherently unstable and feed into the flame-acoustic interaction cycle. The receptivity of the nonreacting shear layer to prevalent acoustic forcing in terms of development of coherent structures and instability of different hydrodynamic modes in the recirculation are detailed. The case of reacting flow instabilities involves several mechanisms of flame-acoustic coupling, such as vortex combustion; vortex-wall interactions; vortex-vortex interactions; flame area fluctuations and equivalence ratio, vorticity, and entropy fluctuations; bifurcations of the combustor dynamics; and its Strouhal scaling. In some instances, flame blowout dynamics as well as flashback are coupled with the prevalent acoustic oscillations. Some of these aspects have been well diagnosed in the recent literature, notably using planar laser-induced fluorescence for flame marking and particle image velocimetry to characterize the flow in a time-resolved manner. Reduced order models based on empirically determined flame transfer functions and predictive tools for onset of combustion instability based on time series analysis, either from a nonlinear dynamical viewpoint or a data-driven approach, are in current vogue. © 2017 Taylor & Francis.

Combustion Science and Technology

Dynamics and Diagnostics of Flame-Acoustic Interactions

Combustion instability poses a negative impact on the performance and structural durability of both land-based and aircraft gas turbine engines, and early detection of combustion instabilities is of paramount importance not only for performance monitoring and fault diagnosis, but also for initiating efficient decision and control of such engines. Combustion instability is, in general, characterized by self-sustained growth of large-amplitude pressure tones that are caused by a positive feedback arising from complex coupling of localized hydrodynamic perturbations, heat energy release, and acoustics of the combustor. This paper proposes a fast dynamic data-driven method for detecting early onsets of thermo-acoustic instabilities, where the underlying algorithms are built upon the concepts of symbolic time series analysis (STSA) via generalization of D-Markov machine construction. The proposed method captures the spatiotemporal co-dependence among time series from heterogeneous sensors (e.g. pressure and chemiluminescence) to generate an information-theoretic precursor, which is uniformly applicable across multiple operating regimes of the combustion process. The proposed method is experimentally validated on the time-series data, generated from a laboratory-scale swirl-stabilized combustor, while inducing thermo-acoustic instabilities for various protocols (e.g. increasing Reynolds number (Re) at a constant fuel flow rate and reducing equivalence ratio at a constant air flow rate) at varying air-fuel premixing levels. The underlying algorithms are developed based on D-Markov entropy rates, and the resulting instability precursor measure is rigorously compared with the state-of-the-art techniques in terms of its performance of instability prediction, computational complexity, and robustness to sensor noise. © 2016, © The Author(s) 2016.

Dynamic data-driven prediction of instability in a swirl-stabilized combustor

In this paper, we show how the phenomenon of intermittency observed in systems with turbulent flow-sound interaction is related to the formation of homoclinic orbits in the phase space. Such orbits that emerge via the intersection of the stable and unstable manifold of an equilibrium configuration result from interactions that happen at multiple spatial/temporal scales associated with turbulent convection and wave propagation. Through a quantification of the time spent by the dynamics in the aperiodic states using recurrence plots, we show how the presence of homoclinic orbits in the dynamics may be convincingly demonstrated, which is often not possible through a visual inspection of the phase space of the attractor. © 2013 AIP Publishing LLC.

Identifying homoclinic orbits in the dynamics of intermittent signals through recurrence quantification

Combustion noise has been traditionally thought of as stochastic fluctuations present in the background of the dynamics in combustors amongst the flow, heat release and the chamber acoustics. Through a series of determinism tests, we show that these aperiodic fluctuations are in fact chaotic of moderately high dimensions (d0 8-10). These chaotic fluctuations then transition to high amplitude combustion instability when the operating conditions are varied towards leaner equivalence ratios. Precursors to such a transition from chaos to dynamics dominated by periodic oscillations are of interest to designers and operators of combustors in estimating the boundaries of operability. We introduce a test for chaos, known as 0-1 test for chaos in the literature, as a measure of the proximity of the combustor to an impending instability. The measure is robust and shows a smooth transition for variation in flow conditions towards instability enabling thresholds to be set for operational boundaries.

Loss of chaos in combustion noise as a precursor of impending combustion instability

Experimental data on acoustic pressure measurements obtained over a wide range of conditions is reported for two simple geometries that are commonly studied for their combustion dynamics behaviour. These geometries are the confined bluff-body and the confined backward-facing steps. The data indicate regimes of flow-acoustic lock-on that signifies the onset of combustion instability, marked by the excitation of high-amplitude discrete tones of sound in the combustor. The high-speed chemiluminescence imaging of the combustion zone indicates heat-release-rate fluctuations occurring at the same frequencies as observed in the acoustic spectra. Attention is then devoted to the data obtained under cold-flow conditions to illustrate distinctly different behaviour than when combustion instability occurs, contrary to the commonly held view that the combustion process does not alter the underlying fluid mechanical processes under low-Mach number conditions. © Printed in India.

Sadhana - Academy Proceedings in Engineering Sciences

Vortex-acoustic lock-on in bluff-body and backward-facing step combustors

Signal Processing

State splitting and merging in probabilistic finite state automata for signal representation and analysis

Volume 4B: Combustion, Fuels and Emissions

Engineering Precursors to Forewarn the Onset of an Impending Combustion Instability

Online detection of impending instability in a combustion system using tools from symbolic time series analysis

Journal	International Journal of Spray and Combustion Dynamics
Publisher	Multi-Science Publishing Co. Ltd
ISSN	17568277
Open Access	No